Since reading your post I've tried toggling the QOS and WMM settings. I have not noticed any increase in performance in my environment, but I'm glad its helped you.
I did notice that toggling off WMM will often cause the router web interface to crash. I turn off WMM click SAVE and the router does its soft reboot. Upon restart the router passes traffic OK, but I can no longer login to the web admin for the router. I am forced to pull the power cord to restore functionality of the web interface.
I've now tested this several times and confirmed it occurs everytime I turn off WMM and SAVE.
Yes, V2.02NA firmware.